Birthweight is an important indicator of the approximate maturity of a newborn infant and the ability of that newborn infant to survive. The birthweight of an infant is dependent on the duration of the pregnancy and its rate of fetal growth. Infants who are delivered earlier than normal are expected to be of smaller birthweight than average. Additionally, infants who had slower or faster fetal growth can also have lower or higher than usual birthweights. Figure 1 portrays the birthweight distribution of singleton live births (babies born singly) to U.S. resident mothers from 1995 to 1997. The graph reveals a somewhat bell-shaped distribution with most births (about 80%) concentrated between 2,750 and 4,250 grams (between 6 pounds and 9 pounds, 4 ounces). The median birth-weight for U.S. singleton, full-term (forty weeks of gestation) births is nearly 3,500 grams (7 pounds, 11 ounces).
The close relationship between an infant's birth-weight and the risk of dying within the first year of life has long been recognized, and birthweight is often used by researchers as a measure of mortality risk. At light and heavy birthweights, an infant's risk of mortality soars (see Figure 1), although in recent decades, heavier infant births have become less associated with high mortality risks, probably because of medical intervention. Nevertheless, very light infants continue to be at grave risk of mortality, morbidity (disease), and long-term developmental problems.
Populations with more infants born at very high or very low birthweights predictably have higher infant mortality rates. Therefore, it is an established procedure to take birthweight into account when making comparisons of mortality among newborn populations. Whether the comparison involves temporal, geographic, socioeconomic, hospital, or other contrasts, infant mortality differences are typically examined within birthweight categories. Investigations of improving trends in infant mortality rates often start with an examination of the extent to which any changes are related to improvements in the distribution of birthweights within categories (e.g., fewer births at extreme birthweights), as opposed to reductions in birthweight-specific mortality rates (e.g., infants in specific birthweight categories having better survival).
Nearly all of the decline in infant mortality rates in the United States in the last quarter of the twentieth century was due to improvements in survival rather than any improvement in the birthweight distribution. Better survival within birthweight groups has been attributed to advances in obstetric and newborn medical care. The increasing medical care costs that have accompanied these advances, however, raise concerns about overly relying on medical technology to reduce infant death rates. Accordingly, research attention has been directed at finding the determinants of low birthweight in order to develop more cost-effective, population-wide programs to further diminish infant mortality.
Variations of average birthweight have been associated with infant gender, multiple birth factors, and maternal factors, such as race and ethnicity, size, nutrition, and current and previous pregnancy medical risk characteristics. One of the unresolved questions among researchers is whether there is a single common average human birthweight or whether there are normal variations in average birthweight among population subgroups. This question entails important medical care, public health policy, and political aspects as it engenders debate about what is a "normal" birthweight, what is a "high-risk" birthweight, and whether a single "one-size-fits-all" criteria for high-risk birthweights is equally valid for all infants.
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